1. Field of the Invention
This invention relates, in general, to optical printheads and, more specifically, to LED printheads for use in copiers, duplicators, printers, and like devices which produce hard copies.
2. Description of the Prior Art
Optical printheads are used in copiers, duplicators, and printers to expose a photoconductive surface or film in the apparatus in such a manner that a latent image is formed on the film. The image is then developed and transferred to paper for producing a hard copy output from the apparatus. Many optical printheads use light emitting diode (LED) arrays to generate the radiation or light necessary to expose the photoconductive film. The light from the LED's is focused by a lens system upon the surface of the film, and the LED's are selectively controlled electronically to produce the amount of light necessary to construct an image on the photoconductive film.
Linear fiber optic lens arrays are usually constructed with a fiber glass reinforced case or enclosure around the optical fibers. While such an enclosure sufficiently supports and fixes the location of the fibers relative to each other, the overall dimensions of the array are not exact enough for use with high resolution systems, such as an LED printhead. In addition to dimensional variation between lens arrays, the arrays are not usually straight enough to be usable without some method of stiffening and straightening the array.
The dimension of the lens array which is parallel to the axis of the individual fibers is typically on the order of .+-. approximately 5.5%. With a lens length of 16.3 mm, this translates to a tolerance of .+-.0.9 mm. Very accurate focusing cannot be achieved in apparatus which uses the design lens length for positioning because of the large tolerance of that dimension. The center of the lens length is more important in many alignment procedures than the overall length of the lens array. However, because of the dimensional inaccuracy of typical lens arrays, finding the center requires more than measuring or locating a known distance from an end of the lens.
To overcome these problems, various prior art arrangements have been used to compensate for the wide tolerances. U.S. Pat. Nos. 4,059,345 and 4,147,412, issued on Nov. 22, 1977 and Apr. 3, 1979, respectively, address the problem of lens location in electrophotographic apparatus. These patents disclose a mounting arrangement (FIG. 5) wherein the lens has grooves located at its center. The grooves align and hold the lens to fixed mounts permanently located between the image and object planes. Another embodiment (FIG. 4) of these patents requires a hole at the center of the lens to align the lens accurately between the image and object planes. A third embodiment (FIG. 3) uses a sleeve around the lens array which extends below the lens a distance necessary to locate the center of the lens at a desired position. U.S. Pat. No. 4,653,894, issued on Mar. 31, 1987, uses pins which extend into locating holes in the lens array to mount and position the lens in copy machines.
Since the center of the lens array needs to be located at a precise point to properly focus the lens, it is important that establishment and use of the center of the lens be accomplished as easily as possible. Therefore, it is desirable, and it is an object of this invention, to provide a printhead having a lens array which can be easily aligned because the center of the lens is readily located. It is also desirable, and another object of this invention, to provide a printhead having a lens array which is structurally straighter and stiffer than conventional lens arrays.